Behavioral rhythmicity of mice lacking AhR and attenuation of light-induced phase shift by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Transcription factors belonging to the Per/Arnt/Sim (PAS) domain family are highly conserved and many are involved in circadian rhythm regulation. One member of this family, aryl hydrocarbon receptor (AhR), is an orphan receptor whose physiological role is unknown. Recent findings have led to the hypothesis that AhR has a role in circadian rhythm, which is the focus of the present investigation. First, time-of-day-dependent mRNA expression of AhR and its signaling target, cytochrome p4501A1 (Cyp1a1), was determined in C57BL/6J mice by quantitative RT-PCR. Circadian expression of AhR and Cyp1a1 was observed both in the suprachiasmatic nucleus (SCN) and liver. Next, the circadian phenotype of mice lacking AhR (AhRKO) was investigated using behavioral monitoring. Intact AhRKO mice had robust circadian rhythmicity with a similar tau under constant conditions compared to wild-type mice, but a significant difference in tau was observed between genotypes in ovariectomized female mice. Time to reentrainment following 6-h advances or delays of the light/dark cycle was not significantly different between genotypes. However, mice exposed to the AhR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 1 microg/kg of body weight) displayed decreased phase shifts in response to light and had altered expression of Per1 and Bmal1. These results suggest that chronic activation of AhR may affect the ability of the circadian timekeeping system to adjust to alterations in environmental lighting by affecting canonical clock genes. Further studies are necessary to decipher the mechanism of how AhR agonists could disrupt light-induced phase shifts. If AhR does have a role in circadian rhythm, it may share redundant roles with other PAS domain proteins and/or the role of AhR may not be exhibited in the behavioral activity rhythm, but could be important elsewhere in the peripheral circadian system.

2,3,7,8-Tetrachlorodibenzo-p-dioxin exposure prevents cardiac valve formation in developing zebrafish

Cardiovascular malformations are one of the most common congenital birth defects observed in humans. Defects in cardiac valves disrupt normal blood flow. Zebrafish are an outstanding experimental model for studying the effects that environmental contaminants have on developmental processes. Previous research has shown that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) causes blood regurgitation in the heart and reduces peripheral blood flow in embryonic zebrafish, suggesting some form of valve failure. To test this we used video microscopy to examine valve function and structure in developing zebrafish exposed to TCDD. TCDD exposure produced blood regurgitation at both the atrioventricular (AV) and bulboventricular (BV) junctions. In marked contrast to control embryos exposed to the vehicle dimethyl sulfoxide, embryos exposed to TCDD failed to form valve leaflets as the heart matured. In addition, whereas TCDD did not block initial formation of the bulbus arteriosus, we found that TCDD exposure prevented the normal growth and development of this portion of the outflow tract. TCDD altered the localization of endothelial cells at the AV and BV junctions and altered the localized expression of mRNAs bmp4 and notch1b normally associated with the nascent valves. Taken together, our results demonstrate that although TCDD does not prevent the initial specification of the presumptive valve locations, TCDD exposure produces severe alterations in valve development, leading to blood regurgitation and failing circulation in the developing zebrafish.

Dose- and route-dependent teratogenicity, toxicity, and pharmacokinetic profiles of the hedgehog signaling antagonist cyclopamine in the mouse

The Hedgehog (Hh) signaling pathway is an essential regulator of embryonic development and appears to play important roles in postnatal repair and cancer progression and metastasis. The teratogenic Veratrum alkaloid cyclopamine is a potent Hh antagonist and is used experimentally both in vitro and in vivo to investigate the role of Hh signaling in diverse biological processes. Here, we set out to establish an administration regimen for cyclopamine-induced teratogenicity in the mouse. The dysmorphogenic concentration of cyclopamine was determined in vitro via mouse whole-embryo culture assays to be 2.0 microM. We administered cyclopamine to female C57BL/6J mice at varied doses by oral gavage, ip injection, or osmotic pump infusion and assessed toxicity and pharmacokinetic (PK) models. Bolus administration was limited by toxicity and rapid clearance. In vivo cyclopamine infusion at 160 mg/kg/day yielded a dam serum steady-state concentration of approximately 2 microM with a corresponding amniotic fluid concentration of approximately 1.5 microM. Gross facial defects were induced in 30% of cyclopamine-exposed litters, with affected embryos exhibiting cleft lip and palate. This is the first report describing the PKs and teratogenic potential of cyclopamine in the mouse and demonstrates that transient Hh signaling inhibition induces facial clefting anomalies in the mouse that mimic common human birth defects.

The aryl hydrocarbon receptor (AhR) inhibits vanadate-induced vascular endothelial growth factor (VEGF) production in TRAMP prostates

Hypoxia-inducible factor-1 alpha (HIF-1alpha) and aryl hydrocarbon receptor nuclear translocator (ARNT) are basic helix-loop-helix/per-arnt-sim (PAS) family transcription factors. During angiogenesis and tumor growth, HIF-1alpha dimerizes with ARNT, inducing expression of many genes, including vascular endothelial growth factor (VEGF). ARNT also dimerizes with the aryl hydrocarbon receptor (AhR). AhR-null (Ahr(-/-)) transgenic adenocarcinoma of the mouse prostate (TRAMP) mice develop prostate tumors with greater frequency than AhR wild-type (Ahr(+/+)) TRAMP mice, even though prevalence of prostate epithelial hyperplasia is not inhibited. This suggests that Ahr inhibits prostate carcinogenesis. In TRAMP mice, prostatic epithelial hyperplasia results in stabilized HIF-1alpha, inducing expression of VEGF, a prerequisite for tumor growth and angiogenesis. Since ARNT is a common dimerization partner of AhR and HIF-1alpha, we hypothesized that the AhR inhibits prostate tumor formation by competing with HIF-1alpha for ARNT, thereby limiting VEGF production. Prostates from Ahr(+/+), Ahr(+/-) and Ahr(-/-) C57BL/6J TRAMP mice were cultured in the presence of graded concentrations of vanadate, an inducer of VEGF through the HIF-1alpha-ARNT pathway. Vanadate induced VEGF protein in a dose-dependent fashion in Ahr(+/-) and Ahr(-/-) TRAMP cultures, but not in Ahr(+/+) cultures. However, vanadate induced upstream proteins in the phosphatidylinositol 3-kinase-signaling cascade to a similar extent in TRAMPs of each Ahr genotype, evidenced by v-akt murine thymoma viral oncogene homolog (Akt) phosphorylation. These findings suggest that AhR sequesters ARNT, decreasing interaction with HIF-1alpha reducing VEGF production. Since VEGF is required for tumor vascularization and growth, these studies further suggest that reduction in VEGF correlates with inhibited prostate carcinogenesis in Ahr(+/+) TRAMP mice.

Comparative genomics identifies genes mediating cardiotoxicity in the embryonic zebrafish heart

Retinoic acid (RA) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) activate distinct ligand-dependent transcription factors, and both cause cardiac malformation and heart failure in zebrafish embryos. We hypothesized that they cause this response by hyperactivating a common set of genes critical for heart development. To test this, we used microarrays to measure transcript changes in hearts isolated from zebrafish embryos 1, 2, 4, and 12 h after exposure to 1 muM RA. We used hierarchical clustering to compare the transcriptional responses produced in the embryonic heart by RA and TCDD. We could identify no early responses in common between the two agents. However, at 12 h both treatments produced a dramatic downregulation of a common cluster of cell cycle progression genes, which we term the cell cycle gene cluster. This was associated with a halt in heart growth. These results suggest that RA and TCDD ultimately trigger a common transcriptional response associated with heart failure, but not through the direct activation of a common set of genes. Among the genes rapidly induced by RA was Nr2F5, a member of the COUP-TF family of transcriptional repressors. We found that induction of Nr2F5 was both necessary and sufficient for the cardiotoxic response to RA.

Efficacy of dose regimen and observation of herd immunity from a vaccine against Escherichia coli O157:H7 for feedlot cattle

A clinical trial was conducted to test the effect of a vaccine product containing type III secreted proteins of Escherichia coli O157:H7 on the probability that feedlot steers shed E. coli O157:H7 in feces. Six hundred eight same-source steers were utilized. Of these, 480 steers were assigned randomly to 60 pens (eight head per pen) and to one of four vaccination treatments (120 cattle per treatment, two head per treatment per pen). The four treatments were (i) no vaccination; (ii) one dose, vaccinated once at reimplant (day 42); (iii) two doses, vaccinated on arrival (day 0) and again at reimplant (day 42); and (iv) three doses, vaccinated on arrival (day 0), on day 21, and again at reimplant (day 42). The remaining 128 steers were assigned randomly to 12 pens within the same feedlot to serve as unvaccinated external controls. The probability of detecting E. coli O157:H7 among cattle receiving different doses of vaccine was compared with that of unvaccinated external control cattle, accounting for clustering by repeated measures, block, and pen and fixed effects of vaccine, corn product, and test period. Vaccine efficacy of receiving one, two, and three doses of vaccine was 68, 66, and 73%, respectively, compared with cattle in pens not receiving vaccine. Cattle receiving three doses of vaccine were significantly less likely to shed E. coli O157:H7 than unvaccinated cattle within the same pen. Unvaccinated cattle housed with vaccinated cattle were 59% less likely to shed E. coli O157:H7 than cattle in pens not receiving vaccine, likely because they benefited from herd immunity. This study supports the hypothesis that vaccination with this vaccine product effectively reduces the probability for cattle to shed E. coli O157:H7. There was no indication that the vaccine affected performance or carcass quality. In addition, we found that vaccinating a majority of cattle within a pen offered a significant protective effect (herd immunity) to unvaccinated cattle within the same pen.

Effect of a vaccine product containing type III secreted proteins on the probability of Escherichia coli O157:H7 fecal shedding and mucosal colonization in feedlot cattle

Preharvest intervention strategies to reduce Escherichia coli O157:H7 in cattle have been sought as a means to reduce human foodborne illness. A blinded clinical trial was conducted to test the effect of a vaccine product on the probability that feedlot steers, under conditions of natural exposure, shed E. coli O157:H7 in feces, are colonized by this organism in the terminal rectum, or develop a humoral response to the respective antigens. Steers (n = 288) were assigned randomly to 36 pens (eight head per pen), and pens were randomized to vaccination treatment in a balanced fashion within six dietary treatments of an unrelated nutrition study. Treatments included vaccination or placebo (three doses at 3-week intervals). Fecal samples for culture (n = 1,410) were collected from the rectum of each steer on pretreatment day 0 and posttreatment days 14, 28, 42, and 56. Terminal rectum mucosal (TRM) cells were aseptically collected for culture at harvest (day 57 posttreatment) by scraping the mucosa 3.0 to 5.5 cm proximal to the rectoanal junction. E. coli O157:H7 was isolated and identified with selective enrichment, immunomagnetic separation, and PCR confirmation. Vaccinated cattle were 98.3% less likely to be colonized by E. coli O157:H7 in TRM cells (odds ratio = 0.014, P < 0.0001). Diet was also associated with the probability of cattle being colonized (P = 0.04). Vaccinated cattle demonstrated significant humoral responses to Tir and O157 lipopolysaccharide. These results provide evidence that this vaccine product reduces E. coli O157:H7 colonization of the terminal rectum of feedlot beef cattle under conditions of natural exposure, a first step in its evaluation as an effective intervention for food and environmental safety.

Noggin is required for normal lobe patterning and ductal budding in the mouse prostate

Mesenchymal expression of the BMP antagonist NOGGIN during prostate development plays a critical role in pre-natal ventral prostate development and opposes BMP4-mediated inhibition of cell proliferation during postnatal ductal development. Morphologic examination of newborn Noggin-/- male fetuses revealed genitourinary anomalies including cryptorchidism, incomplete separation of the hindgut from the urogenital sinus (UGS), absence of the ventral mesenchymal pad, and a complete loss of ventral prostate (VP) budding. Examination of lobe-specific marker expression in the E14 Noggin-/- UGS rescued by transplantation under the renal capsule of a male nude mouse confirmed a complete loss of VP determination. More modest effects were observed in the other lobes, including decreased number of ductal buds in the dorsal and lateral prostates of newborn Noggin-/- males. BMP4 and BMP7 have been shown to inhibit ductal budding and outgrowth by negatively regulating epithelial cell proliferation. We show here that NOGGIN can neutralize budding inhibition by BMP4 and rescues branching morphogenesis of BMP4-exposed UGS in organ culture and show that the effects of BMP4 and NOGGIN activities converge on P63+ epithelial cells located at nascent duct tips. Together, these studies show that the BMP-NOGGIN axis regulates patterning of the ventral prostate, regulates ductal budding, and controls proliferation of P63+ epithelial cells in the nascent ducts of developing mouse prostate.

Past radioactive particle contamination in the Columbia river at the Hanford site, USA

One closed-loop and eight single-pass plutonium production reactors originally operated on the Columbia river. During the 26 years of single-pass reactor operations, small amounts of radioactive particles were released in liquid discharges to the Columbia river and were deposited in sediment and cobble along the shoreline and on islands in the river. Islands located adjacent to D island and immediately downstream of D island had the greatest density of particles. In 1979, the small particles contained between 63 and 890 kBq of cobalt-60 activity. Dose rates emanating from those particles ranged from 1 to 14 microGy h(-1). Because of the short half-life of cobalt-60 (5.3 y), the hot-particle problem at Hanford has taken care of itself through radiological decay. Some scientists have proposed that it is economically and environmentally advantageous to manage isolated low-level contaminated sites with institutional controls until the activity decays and the sites can be released rather than to pursue expensive clean-up options.

Identification and Characterization of Several Dietary Alkaloids as Weak Inhibitors of Hedgehog Signaling

The Hedgehog (Hh) signaling pathway plays an integral role in the patterning and development of diverse structures in the vertebrate embryo. Aberrations in Hh signaling are associated with a range of developmental defects including failure of interhemispheric division of the embryonic forebrain as well as midline facial dysmorphia including cleft lip/palate and cyclopia, collectively termed holoprosencephaly (HPE). Postnatally, Hh signaling has been postulated to play a pivotal role in healing and repair processes and inappropriate Hh pathway activation has been implicated in several types of cancers. The Veratrum alkaloid cyclopamine is a potent inhibitor of Hh signaling and causes HPE-like defects in diverse species including sheep, hamster, mouse, and zebra fish. Using murine cell-based assays, we have determined that a number of dietary alkaloids similar in structure to cyclopamine also inhibit Hh signaling but with significantly lower potency. We found that these dietary compounds act additively through a mechanism similar to cyclopamine, downstream of Ptc1 and upstream of Gli1. Using an embryonic zebra fish developmental assay, we found that while cyclopamine exposure caused HPE-like defects, exposure to one of these dietary compounds, solanidine, did not.

Developmental toxicity of low generation PAMAM dendrimers in zebrafish

Biological molecules and intracellular structures operate at the nanoscale; therefore, development of nanomedicines shows great promise for the treatment of disease by using targeted drug delivery and gene therapies. PAMAM dendrimers, which are highly branched polymers with low polydispersity and high functionality, provide an ideal architecture for construction of effective drug carriers, gene transfer devices and imaging of biological systems. For example, dendrimers bioconjugated with selective ligands such as Arg-Gly-Asp (RGD) would theoretically target cells that contain integrin receptors and show potential for use as drug delivery devices. While RGD-conjugated dendrimers are generally considered not to be cytotoxic, there currently exists little information on the risks that such materials pose to human health. In an effort to compliment and extend the knowledge gleaned from cell culture assays, we have used the zebrafish embryo as a rapid, medium throughput, cost-effective whole-animal model to provide a more comprehensive and predictive developmental toxicity screen for nanomaterials such as PAMAM dendrimers. Using the zebrafish embryo, we have assessed the developmental toxicity of low generation (G3.5 and G4) PAMAM dendrimers, as well as RGD-conjugated forms for comparison. Our results demonstrate that G4 dendrimers, which have amino functional groups, are toxic and attenuate growth and development of zebrafish embryos at sublethal concentrations; however, G3.5 dendrimers, with carboxylic acid terminal functional groups, are not toxic to zebrafish embryos. Furthermore, RGD-conjugated G4 dendrimers are less potent in causing embryo toxicity than G4 dendrimers. RGD-conjugated G3.5 dendrimers do not elicit toxicity at the highest concentrations tested and warrant further study for use as a drug delivery device.

Lrrc10 is required for early heart development and function in zebrafish

Leucine-rich Repeat Containing protein 10 (LRRC10) has recently been identified as a cardiac-specific factor in mice. However, the function of this factor remains to be elucidated. In this study, we investigated the developmental roles of Lrrc10 using zebrafish as an animal model. Knockdown of Lrrc10 in zebrafish embryos (morphants) using morpholinos caused severe cardiac morphogenic defects including a cardiac looping failure accompanied by a large pericardial edema, and embryonic lethality between day 6 and 7 post fertilization. The Lrrc10 morphants exhibited cardiac functional defects as evidenced by a decrease in ejection fraction and cardiac output. Further investigations into the underlying mechanisms of the cardiac defects revealed that the number of cardiomyocyte was reduced in the morphants. Expression of two cardiac genes was deregulated in the morphants including an increase in atrial natriuretic factor, a hallmark for cardiac hypertrophy and failure, and a decrease in cardiac myosin light chain 2, an essential protein for cardiac contractility in zebrafish. Moreover, a reduced fluorescence intensity from NADH in the morphant heart was observed in live zebrafish embryos as compared to control. Taken together, the present study demonstrates that Lrrc10 is necessary for normal cardiac development and cardiac function in zebrafish embryos, which will enhance our understanding of congenital heart defects and heart disease.

Effect of Lactobacillus acidophilus strain NP51 on Escherichia coil O157:H7 fecal shedding and finishing performance in beef feedlot cattle

A 2-year study was conducted during the summer months (May to September) to test the effectiveness of feeding Lactobacillus acidophilus strain NP51 on the proportion of cattle shedding Escherichia coli O157:H7 in the feces and evaluate the effect of the treatment on finishing performance. Steers (n = 448) were assigned randomly to pens, and pens of cattle were assigned randomly to NP51 supplementation or no supplementation (control). NP51 products were mixed with water and applied as the feed was mixed daily in treatment-designated trucks at the rate of 10(9) CFU per steer. Fecal samples were collected (n = 3,360) from the rectum from each animal every 3 weeks, and E. coli O157:H7 was isolated by standard procedures, using selective enrichment, immunomagnetic separation, and PCR confirmation. The outcome variable was the recovery of E. coli O157:H7 from feces, and was modeled using logistic regression accounting for year, repeated measures of pens of cattle, and block. No significant differences were detected for gain, intakes, or feed efficiency of control or NP51-fed steers. The probability for cattle to shed E. coli O157:H7 varied significantly between 2002 and 2003 (P = 0.004). In 2002 and 2003, the probability for NP51-treated steers to shed E. coli O157:H7 over the test periods was 13 and 21%, respectively, compared with 21 and 28% among controls. Over the 2 years, NP51-treated steers were 35% less likely to shed E. coli O157: H7 than were steers in untreated pens (odds ratio = 0.58, P = 0.008). This study is consistent with previous reports that feeding NP51 is effective in reducing E. coli O157:H7 fecal shedding in feedlot cattle.

The aryl hydrocarbon receptor inhibits prostate carcinogenesis in TRAMP mice

The aryl hydrocarbon receptor (AhR) is a transcription factor that mediates the inhibitory effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on prostate growth and also modulates normal prostate development. This is evidenced by AhR null mice (Ahr-/-) having smaller dorsolateral and anterior prostates, even though all prostate lobes remain histologically normal. To test the hypothesis that loss of the AhR increases the rate of prostate carcinogenesis, the incidence of macroscopic prostate tumors was determined in Ahr+/+, Ahr+/- and Ahr-/- C57BL/6J transgenic adenocarcinoma of the mouse prostate (TRAMP) mice at 35, 70, 105, 140, 175 and 210 days of age. From 140 days, prostate tumor incidence was greater in Ahr-/- (60%) and Ahr+/- (43%) mice than in Ahr+/+ mice (16%). Allele quantification did not indicate a loss of the wild-type Ahr allele in heterozygous TRAMP tumors, suggesting that tumor formation in these mice was not due to a loss of Ahr heterozygosity. Prostatic SV40 large T antigen mRNA expression and protein localization were comparable in TRAMP mice of each Ahr genotype. Prostates from all mice of each Ahr genotype were histologically indistinguishable, exhibiting diffuse epithelial hyperplasia by 105 days of age. mRNA expression and protein localization for molecular markers of neuroendocrine differentiation, including chromogranin A and neuropilin-1, were elevated in prostate tumors compared to tumor-free ventral prostates, regardless of Ahr genotype or age. Taken together, these results demonstrate that the Ahr inhibits prostate carcinogenesis in C57BL/6J TRAMP mice by interfering with neuroendocrine differentiation.

Blocking expression of AHR2 and ARNT1 in zebrafish larvae protects against cardiac toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin

The zebrafish (Danio rerio) has become an attractive vertebrate model for studying developmental processes, and is emerging as a model system for studying the mechanisms by which xenobiotic compounds perturb normal development. Embryos treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) shortly after fertilization exhibit a range of adverse effects on the heart: an early reduction in cardiac myocyte number, followed by a change in heart looping and morphology, with an apparent compaction of the ventricle and overall decrease in heart size. These changes are accompanied by impaired cardiac function including a decrease in cardiac output and eventually irreversible ventricular standstill. The mechanisms involved in mediating effects of TCDD on the heart remain unknown. However, it is widely accepted that aryl hydrocarbon receptor (AHR) activation mediates endpoints of TCDD toxicity in vertebrates. In zebrafish, there are multiple forms of AHR and AHR nuclear translocator protein (ARNT) raising the question about whether different endpoints of TCDD toxicity are mediated by different components of the AHR/ARNT pathway. To address this question we used morpholino oligonucleotide technology to specifically block the expression of zfAHR2, zfARNT1, zfARNT2, and zfCYP1A, and assessed the previously described effects of TCDD on heart morphology, size, and function in the developing morphants. We report that blocking zfAHR2 and zfARNT1 expression provided protection against the TCDD-mediated alteration in heart morphology, reduced cardiac myocyte number, decreased cardiac output and ventricular standstill in zebrafish larvae, while the zfarnt2 and zfcyp1a morpholinos did not block the TCDD-induced cardiac toxicity.

The 2005 World Health Organization reevaluation of human and Mammalian toxic equivalency factors for dioxins and dioxin-like compounds

In June 2005, a World Health Organization (WHO)-International Programme on Chemical Safety expert meeting was held in Geneva during which the toxic equivalency factors (TEFs) for dioxin-like compounds, including some polychlorinated biphenyls (PCBs), were reevaluated. For this reevaluation process, the refined TEF database recently published by Haws et al. (2006, Toxicol. Sci. 89, 4-30) was used as a starting point. Decisions about a TEF value were made based on a combination of unweighted relative effect potency (REP) distributions from this database, expert judgment, and point estimates. Previous TEFs were assigned in increments of 0.01, 0.05, 0.1, etc., but for this reevaluation, it was decided to use half order of magnitude increments on a logarithmic scale of 0.03, 0.1, 0.3, etc. Changes were decided by the expert panel for 2,3,4,7,8-pentachlorodibenzofuran (PeCDF) (TEF = 0.3), 1,2,3,7,8-pentachlorodibenzofuran (PeCDF) (TEF = 0.03), octachlorodibenzo-p-dioxin and octachlorodibenzofuran (TEFs = 0.0003), 3,4,4',5-tetrachlorbiphenyl (PCB 81) (TEF = 0.0003), 3,3',4,4',5,5'-hexachlorobiphenyl (PCB 169) (TEF = 0.03), and a single TEF value (0.00003) for all relevant mono-ortho-substituted PCBs. Additivity, an important prerequisite of the TEF concept was again confirmed by results from recent in vivo mixture studies. Some experimental evidence shows that non-dioxin-like aryl hydrocarbon receptor agonists/antagonists are able to impact the overall toxic potency of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds, and this needs to be investigated further. Certain individual and groups of compounds were identified for possible future inclusion in the TEF concept, including 3,4,4'-TCB (PCB 37), polybrominated dibenzo-p-dioxins and dibenzofurans, mixed polyhalogenated dibenzo-p-dioxins and dibenzofurans, polyhalogenated naphthalenes, and polybrominated biphenyls. Concern was expressed about direct application of the TEF/total toxic equivalency (TEQ) approach to abiotic matrices, such as soil, sediment, etc., for direct application in human risk assessment. This is problematic as the present TEF scheme and TEQ methodology are primarily intended for estimating exposure and risks via oral ingestion (e.g., by dietary intake). A number of future approaches to determine alternative or additional TEFs were also identified. These included the use of a probabilistic methodology to determine TEFs that better describe the associated levels of uncertainty and "systemic" TEFs for blood and adipose tissue and TEQ for body burden.

Aryl hydrocarbon receptor activation produces heart-specific transcriptional and toxic responses in developing zebrafish

Proper regulation of the aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor, is required for normal vertebrate cardiovascular development. AHR hyperactivation by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) during zebrafish (Danio rerio) development results in altered heart morphology and function, culminating in death. To identify genes that may cause cardiac toxicity, we analyzed the transcriptional response to TCDD in zebrafish hearts. Zebrafish larvae were exposed to TCDD for 1 h at 72 h after fertilization (hpf), and the hearts were extracted for microarray analysis at 1, 2, 4, and 12 h after exposure (73, 74, 76, and 84 h postfertilization). The remaining body tissue was also collected at each time for comparison. TCDD rapidly induced expression in 42 genes within 1 to2hof exposure. These genes function in xenobiotic metabolism, proliferation, heart contractility, and pathways that regulate heart development. Furthermore, these expression changes preceded signs of cardiovascular toxicity, characterized by decreased stroke volume, peripheral blood flow, and a halt in heart growth. This identifies strong candidates for important AHR target genes. It is noteworthy that the TCDD-induced transcriptional response in the hearts of zebrafish larvae was substantially different from that induced in the rest of the body tissues. One of the biggest differences included a cluster of genes that were down-regulated 12 h after exposure in heart tissue, but not in the body samples. More than 70% of the transcripts in this heart-specific cluster promote cellular growth and proliferation. Thus, the developing heart stands out as being responsive to TCDD at both the level of toxicity and gene expression.

Understanding dioxin developmental toxicity using the zebrafish model

Zebrafish (Danio rerio) have advantages over mammals as an animal model for investigating developmental toxicity. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (dioxin, TCDD), a persistent global contaminant, is the most comprehensively studied developmental toxicant in zebrafish. The hallmark responses of TCDD developmental toxicity manifested in zebrafish larvae include edema, anemia, hemorrhage, and ischemia associated with arrested growth and development. Heart and vasculature development and function are severely impaired, and jaw malformations occur secondary to inhibited chondrogenesis. The swim bladder fails to inflate, and the switch from embryonic to adult erythropoiesis is blocked. This profile of developmental toxicity responses, commonly referred to as "blue sac syndrome" because the edematous yolk sac appears blue, is observed in the larval form of all freshwater fish species exposed to TCDD at the embryonic stage of development. Components of the aryl hydrocarbon receptor/aryl hydrocarbon receptor nuclear translocator (AHR/ARNT) signaling pathway in zebrafish have been identified and functionally characterized. Their role in mediating TCDD toxicity has been determined using morpholinos to specifically knockdown the translation of zfAHR1, zfAHR2, zfARNT1, and zfARNT2 mRNAs, respectively, and a line of zfARNT2 null mutant zebrafish has provided further insight. These studies have shown that zfAHR2 and zfARNT1 mediate TCDD developmental toxicity. In addition, the growing use of molecular and genomic tools for research on zebrafish have led to advances in our understanding of the mechanism of TCDD developmental toxicity at the molecular level, including the recent finding that toxicity is not mediated by increased cytochrome P4501A (zfCYP1A) expression.

Growth, morphogenesis, and differentiation during mouse prostate development in situ, in renal grafts, and in vitro

BACKGROUND

In vitro organ culture and renal grafting of the urogenital sinus (UGS) have both been used as models of prostate development. However, neither has been rigorously examined for its fidelity to replicate the canonical process of prostate differentiation in situ.

METHODS

We assessed size, morphology, histology, and the mRNA expression of differentiation marker genes of the E14 male mouse UGS grown for 0-28 days as sub-renal capsule allografts in nude mice or in culture containing androgen and compared these to UGS development in situ.

RESULTS

Development of grafted tissues was morphologically and histologically similar to development in situ but differentiation occurred more rapidly. UGS growth in organ culture resulted in bud formation, but did not trigger cellular differentiation. However, the potential for differentiation was maintained and could be rescued by grafting tissues into nude mice.

CONCLUSIONS

In vitro organ culture and renal grafting of UGS tissues may be appropriate models for studying prostatic bud formation, but only grafting is an appropriate model for prostatic differentiation.

Identification of zebrafish ARNT1 homologs: 2,3,7,8-tetrachlorodibenzo-p-dioxin toxicity in the developing zebrafish requires ARNT1

To use the zebrafish (Danio rerio) as a model to study 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) developmental toxicity, it is essential to know which proteins are involved in mediating toxicity. Previous work has identified zfAHR2 as the receptor that binds TCDD mediating downstream responses. Although zfARNT2b can form a functional heterodimer with zfAHR2 in vitro, zfarnt2 null mutants show no protection against endpoints of TCDD developmental toxicity, demonstrating that zfARNT2b cannot be the physiological dimerization partner for zfAHR2 mediating responses to TCDD in zebrafish embryos. The purpose of the current study was to identify an alternate dimerization partner(s) for zfAHR2 that may function to mediate TCDD developmental toxicity. By searching zebrafish genomic sequence and using the polymerase chain reaction-based rapid amplification of cDNA ends technique, three forms of cDNA that seem to be alternate mRNA splice variants of a zebrafish homolog of ARNT1 were detected. Analysis of the zfARNT1 proteins in vitro demonstrates that the two longest forms of zfARNT1, zfARNT1b and zfARNT1c, can form functional heterodimers with zfAHR2. However, the shortest form, zfARNT1a, seems to be nonfunctional with zfAHR2 in vitro. To determine whether a zfARNT1 protein functions with zfAHR2 in vivo, a morpholino targeted against the 5' end of zfARNT1 (zfarnt1-MO) was used. Injection of the zfarnt1-MO before TCDD treatment significantly decreases the induction of zfCYP1A mRNA and protein. In addition, zfarnt1 morphants show complete protection against TCDD-induced pericardial edema and show partial protection against reduced blood flow and craniofacial malformations caused by TCDD, demonstrating the role of zfARNT1 proteins in mediating these responses.

Altered Prostatic Epithelial Proliferation and Apoptosis, Prostatic Development, and Serum Testosterone in Mice Lacking Cyclin-Dependent Kinase Inhibitors1

Normal prostatic development and some prostatic diseases involve altered expression of the cell-cycle regulators p27 and p21 (also known as CDKN1B and CDKN1A, respectively). To determine the role of these proteins in the prostate, we examined prostatic phenotype and development in mice lacking p27 and/or p21. In p27-knockout (p27KO) mice, epithelial proliferation was increased 2- and 3.8-fold in the ventral and dorsolateral prostate, respectively, versus wild-type (WT) mice, although prostatic weights were not different. Epithelial apoptosis was increased in p27KO mice and may account for the lack of a concurrent increase in weight. Testosterone deficiency observed in this group was not the cause of this increase, because vehicle- and testosterone-treated p27KO mice had similar percentages of apoptotic cells. Also observed was a trend toward a decreased functional epithelial cytodifferentiation, indicating a potential role of p27 in this process. Conversely, dorsolateral prostate and seminal vesicle (SV) of p21-knockout (p21KO) mice, and all prostatic lobes and SV of p21/p27 double-knockout mice, weighed significantly less compared to the WT mice, and their epithelial proliferation was normal. Decreased testosterone concentrations may contribute to the decreased prostatic weights. However, other factors may be involved, because testosterone replacement only partially restored prostatic weights. We conclude that loss of p27 increases prostatic epithelial proliferation and alters differentiation but does not result in prostatic hyperplasia because of increased epithelial cell loss. The p21KO mice showed phenotypes distinctly different from those of p27KO mice, suggesting nonredundant roles of p21 and p27 in prostatic development. Loss of p27 or of both p21 and p27 results in serum testosterone deficiency, complicating analysis of the prostatic effects of these cell-cycle regulators.